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Acids and Alkalis

When a substance dissolves in water,
the solution may be acidic, neutral or alkaline.

What is an Acid?

Arrhenius defined an acid is any substance that produces
H+  ions or H3O+  ions in water.
Compare this with the definition of an alkali or base.
H+  ions are called hydrogen ions,
H3O+ ions are called hydroxonium ions.

You will mostly see acids in reactions forming H+  ions.
In reality, H+ is a single proton, and does not exist on its own.
It always attaches to something.
In water it joins to H2O to form H3O+ ions.

This website will use either H+(aq)  or H3O+(aq)
to mean acid in water.

An acid has a pH of less than 7.
See examples of acids, properties of acids and uses of acids.

See also Lowry and Brønsted.

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